The present invention relates to gain equalizers used to equalize the gain of balanced microwave transistor amplifier configurations, and more particularly to an equalizer which provides the desired gain equalization through the loss of power due to phase error introduced by unequal transmission line lengths connecting the microwave transistor amplifiers to the output power combiner.
Microwave transistor amplifiers are used in both the transmitter and receiver amplifier chains of radar, communications and electronic warfare systems. Microwave transistors used in the amplifiers have a negative gain slope, i.e., the gain decreases as the frequency increases. Because microwave transistor amplifiers are used most commonly as preamplifiers or drivers, this gain slope must usually be compensated. In many applications, the RF drive power as a function of frequency has either a positive or a negative slope and at the same time the output power must have either a positive or a negative slope. In these applications the gain equalization may need be only partial or possibly compensate a positive slope rather than a negative slope usually associated with microwave transistors.
Obtaining both optimum transistor performance (maximum gain, maximum power, minimum junction temperature, etc.) and correct gain equalization has always been a very difficult task, one which has possibly never been achieved. Conventionally, gain equalizers are inserted between amplifier stages. This gain equalizer position usually requires an equalizer which has an impedance equal or nearly equal to the connecting transmission line impedance for the previous stage. This prevents changes in the source impedance for the following stage and changes in the load impedance. In many cases, this rules out purely reflective/mismatch type equalization unless combined with ferrite isolators which absorb the reflections and therefore maintain the load and source impedance for the previous stage and following stage respectively. In many applications the reflective/mismatch type equalization is derived from adjusting the input matching circuit and consequently the output matching circuit of the microwave transistors so as to provide the correct equalization. This scheme is difficult since one must optimize transistor performance while providing gain equalization through the adjustment of the input and consequently output matching circuits. These configurations require more parts, making it more difficult to manufacture and less producible In addition, the insertion loss and (reflection) mismatch of the ferrite circulator may also need to be compensated. Also, ferrite circulators are relatively difficult to design, tune and install, and are subject to significant performance variations over normally encountered environmental temperature excursions.
Four port hybrid devices combined with reflective mismatch type equalization have been used as the input divider to distribute input signals, and absorb the coherent reflections and therefore maintain the load impedance for the previous stage. However, in practice the reflections are difficult to maintain coherent while tuning to optimize other transistor characteristics and therefore the non-coherent reflections are not absorbed and change the load impedance as seen by the previous stage.
Parallel coupled line directional couplers have also been used in the past for gain equalizers. The length of the coupled lines is set equal to a half-wavelength at the frequency where no insertion loss is desired. As the frequency departs from this design frequency, the insertion loss of the coupler increases monotonically. This loss-frequency relationship can be used to offset the gain-frequency relationship of the amplifiers, thereby equalizing the gain over the bandwidth. Relatively tight coupling is required for any reasonable amount of equalization. A typical gain slope over a 12% bandwidth is 1.5dB. This would require an even and odd mode impedance of b 195.3 ohms and 12.8 ohms, respectively. These impedance values required TEM coupled line dimensions which are physically unrealizable in a practical sense. Increasing the length to multiples of half-wavelengths at the frequency of minimum insertion loss increases the sensitivity versus frequency and thereby slightly reduces the tight coupling requirement. Cascading of moderate coupling couplers is a possible solution to obtain a reasonable amount of equalization. However, all of these schemes: half-wavelength, multiple half-wavelength and cascading require either more layout space or are relatively difficult to manufacture.
Power dividers such as two branch hybrids and rat races are not usable for gain equalization in the same way as parallel coupled line directional couplers. These hybrids are not matched where the zero coupling and insertion loss occurs, as is the case for parallel coupled line couplers.
It would therefore represent an advance in the art to provide a gain equalizer which is relatively simple to construct, which does not require any additional loads for absorption of the unwanted signal power, does not significantly increase the insertion loss of the amplifier chain at the critical frequency where minimum insertion loss is desired, does not employ mismatches to degrade the performance of the microwave transistor amplifiers, and does provide a near continuum of monotonic gain equalization.